When browsing through the treasures of the Royal Collection (while unsuccesfully trying to find a hint to the "E"-brooch of Queen Alexandra), I found this wonderful full-length portrait of a jewel bedecked Queen Mary .
Credit The Royal Collection
QM wears the outer parts of the Cockade brooch, connected with (with) brooch, as a stomacher? The RC's collections caption says: "Photograph of full-length portrait of Queen Mary wearing her Coronation crown (without arches), the collier resille made by Cartier for Queen Alexandra and altered by Garrard for Queen Mary in 1926, the Coronaton necklace, two of Queen Victoria's bow brooches hung with Cullinan diamonds and another stomacher, as well as family orders and the Garter star. "
Alas, again no particular mention or information for this interesting brooch (Cockade), I already regretted the fact, that Roberts didn't provided more information about this jewel.